The present invention relates to a process for preparing high-purity 2,2-bis(4-hydroxyphenyl)propane (referred to as bisphenol A hereinafter).
Bisphenol A is used as a raw material for polycarbonate resins and epoxy resins and also for engineering plastics recently. Colorless and high-purity bisphenol A is required for these uses.
Bisphenol A is prepared by the reaction of acetone with excess phenol in the presence of an acidic catalyst. The product mixture contains bisphenol A and also the catalyst, unreacted acetone, unreacted phenol, water, and other by-products such as coloring substances.
Inorganic acids such as hydrochloric acid and strongly acidic ion-exchange resins are known as the catalyst for the reaction.
There are many known processes for obtaining high-purity bisphenol A from the product mixture. For example, in the case where hydrochloric acid has been used as the catalyst in the reaction, the product mixture is heated at 110.degree. to 120.degree. C. under reduced pressure, thereby removing hydrochloric acid, unreacted acetone, water, and a small amount of phenol, and thereafter bisphenol A in the form of an adduct with phenol is separated by cooling. The other process includes distillation to separate bisphenol A from other substances having a higher and lower boiling point than that of bisphenol A. The thus-obtained bisphenol A may be further purified by extraction with a solvent or recrystallization from a solution.
In the case where hydrochloric acid has been used as the catalyst, the product mixture which has been distilled to remove hydrochloric acid, acetone and water, still contains a trace amount of hydrochloric acid which causes some trouble in the subsequent purification steps.
One of the trouble is the corrosion of equipment due to the acid. The corrosion yields metal salts which contaminate bisphenol A, and the removal of the metal salts requires a complicated purification procedure. A possible countermeasure is to use equipment made of an acid-resistant material; however, this is not economical because such equipment is expensive.
Another trouble is that bisphenol A is decomposed due to the acidic substance during distillation, as described in U.S. Pat. 3,073,868 and Japanese Patent Publication No. 4875/1963.
The process in which a strongly acidic ion-exchange resin is used as the catalyst is more favorable than the process in which hydrochloric acid is used as the catalyst because the ion-exchange resin is separated more easily. However, in the case where the reaction is carried out at 70.degree. to 100.degree. C., the product mixture also contains a trace amount of free acid originating from the exchange groups which are eliminated.
GB Patent No. 1,377,227 and Japanese Patent Laid-open No. 1543/1974 disclose that bisphenol A can be distilled without any decomposition, if the distillation is conducted after the adduct of bisphenol A with phenol is melted and the melt is treated with an acidic or weakly basic ion-exchange resin. However, heating above 90.degree. C. is necessary for melting the adduct of bisphenol A with phenol. Therefore, a weakly basic ion-exchange resin having exchange groups of secondary or tertiary amine is not suitable for treating the melt, because the ion-exchange resin can not withstand temperatures above 70.degree.-80.degree. C. for a long period of time. At temperatures above that limit, the exchange groups are eliminated from the weakly basic ion-exchange resin and the eliminated exchange groups discolor the treated solution with a color between yellow and red.